Lossless 48 kHz digital output is a huge deal -- with the right headphones and source material, it will *vastly* affect audio quality. Bravo, Apple.
While the output is analog now, existing iPhones can play up to 24 bit @ 48kHz ALAC files, as far as I can tell (anything higher than that won't sync). And it wouldn't surprise me if digital output has also been available (with a 30-pin adaptor, perhaps).